This invention relates generally to plasma processing and, more particularly, to techniques for generating a plasma in a processing chamber. Plasma processing is widely used in a variety of semiconductor fabrication processes, such as etching, ashing and thin film deposition. Process gases are introduced into a vacuum chamber and a plasma of ions, electrons and other particles is formed from the gases in the chamber. Depending on the specific process being performed, particles in the plasma are accelerated toward a semiconductor wafer or substrate, for deposition or etching. During the processing, the plasma remains in a substantially steady state, although the constituents of the plasma are continuously changing as process gases enter and exit the chamber, gas particles are ionized, ions are recombined with electrons, particles are accelerated out of the plasma, and so forth. An essential feature of plasma processing systems is a mechanism for "activating" the plasma, i.e. for establishing and maintaining the plasma from the process gases that are introduced into the chamber. Typically, the plasma is established and maintained by a radio-frequency (rf) electric field applied to the process gases. However, conventional techniques for establishing and maintaining plasmas in vacuum chambers have significant disadvantages.
In a conventional parallel plate processing chamber, rf power is applied to the chamber through upper and lower parallel-plate electrodes, the lower one of which is typically used to support a semiconductor substrate. The principal drawback of this approach is that the plasma cloud formed between the parallel plates is necessarily in contact, or near contact, with the semiconductor substrate. In most processes, direct exposure of the substrate to the plasma is not desired. Moreover, in plasma processing there is an important distinction between the plasma density, i.e. the density of ions or other active particles in the plasma, and the plasma energy, i.e. the energy of plasma particles impinging on the substrate being processed. Usually it is desirable to be able to control these parameters independently. However, if the electrical power applied to parallel-plate electrodes is increased to raise the plasma density, the effect of the plasma on the substrate will be immediately increased because the substrate is directly exposed to the plasma.
Another conventional plasma processing configuration avoids exposure of the substrate to the plasma by positioning the substrate in a perforated barrel inside the processing chamber. The barrel, sometimes referred to as an etching tunnel, holds a substrate to be etched and the plasma is generated outside the tunnel. This technique has other disadvantages, however. At higher rf powers, the material of the etching tunnel is removed by plasma sputtering and the substrate is subject to contamination. Further, the presence of an etching tunnel makes it difficult to attain uniformity in the processing of large semiconductor wafers.
A more complex plasma generation system is the magnetically enhanced microwave (MEMW) plasma generator. Energy is supplied to the plasma chamber at microwave frequency through a waveguide. A second power generator supplies rf energy through an electrode plate to control the plasma. Magnetic enhancement is applied if a high density plasma is desired, because at microwave frequencies the oscillated electrons in the plasma do not have enough kinetic energy to generate a requisite population of ions for etching. In the presence of an appropriate magnetic field, usually applied through a cylindrical coil around the chamber, the electrons gyrate in resonance with the microwave frequency, a condition referred to as electron cyclotron resonance (ECR). Although the ECR configuration avoids the disadvantages of the other conventional plasma generation systems referred to above, its principal disadvantages are its large size, complexity and cost. Its complexity results in equally complex maintenance procedures and its large size and cost render it unsuitable for many applications.
Another technique for plasma generation uses inductive coupling of rf energy into the vacuum chamber. This, like the magnetically enhanced ECR configuration, has the advantage of "decoupling" the control of plasma density and plasma energy. However, inductively coupled plasma generators also tend to be large, complex and expensive.
It will be appreciated from the foregoing that there is still a need for improvement in plasma generation techniques. Ideally, what is needed is an approach that avoids the disadvantages of parallel-plate and etching tunnel configurations, but also avoids the complexity and cost of magnetically enhanced ECR and inductively coupled systems. The present invention fulfills this need.